Method of making water vapor permeable polymer sheet material

ABSTRACT

Coagulation apparatus for producing thick microporous polyurethane sheet material by passing a layer of polymer solvent composition beneath a close spaced plate with liquid non-solvent fed to the gap between the plate and the surface to produce controlled initiation of the coagulation.

This is a division, of application Ser. No. 329,410, filed Feb. 5, 1973,and now U.S. Pat. No. 3,873,254, which in turn is a division, ofapplication Ser. No. 42,793, filed June 2, 1970, and now U.S. Pat. No.3,729,536.

The present invention relates to methods of making microporous polymermaterials particularly but not exclusively materials free from preformedfibrous sheet reinforcement.

The invention is concerned with microporous polymer materials which areproduced by coagulating layers of polymers extended with liquidvehicles, for example, polymer solutions, gels or colloidal dispersionswith liquid non solvents. It has been suggested that the non solvent canbe applied to the surface of the layer of polymer solutions as a vapour,but this is not feasible for relatively thick layers e.g. 0.5millimeters upwards, or as a spray but this produces an uneven surface.The more usual method is to pass the layer on a support with the suppotuppermost into a bath of the liquid non solvent.

This presents considerable problems in conveying the material throughthe bath especially in a continuous process since the polymer layer mayrequire a substantial period of time e.g. of the order of 1/2 hourbefore the polymer surface can be conveyed around a roller without asurface blemish being produced.

The applicants have observed that attempting to pass the layer on asupport with the layer uppermost into a bath of liquid non solventbrings a number of problems. Thus the passage of the material into thebath of liquid and plant vibrations tend to set up ripples in thesurface of the bath and these can often cause surface patterns whichhave an adverse effect on the appearance of the material and render itunsuitable for use as high grade man-made leather like material.

Thus an object of the present invention is to provide a process adaptedto continuous operation to produce a material of consistently goodappearance.

A further object of the present invention is the utilization of acompact and simple plant for carrying out the said process.

The present invention finds a particular application in the productionof relatively thick polymer layers free from preformed fibrous sheetreinforcement for example from 0.5 millimeters up to as thick as 5millimeters or more and especially to the production of layers having athickness making them suitable for use as shoe upper materials forexample 0.8 millimeters to 1.5 millimeters preferably 0.8 to 1.1millimeters for women's weight shoes and 1.1 to 2.5 millimeterspreferably 1.5 to 1.8 millimeters for men's weight shoes.

In order to produce such materials in vapour permeable or microporousform from a layer on a support of solutions of polymers for examplepolyurethanes, in organic hygroscopic or hydrophilic solvents, forexample N,N dimethylformamide, the said solutions containing microscopicwater soluble particles, for example ground sodium chloride, bycoagulation with liquid non solvents for example water or water solventblends, the layer when immersed support uppermost in the aqueoussolution needs to remain immersed for about 1/2 to 1 hour to achievecoagulation of the polymer to self supporting form when the finishedlayers are to be about 1.5 to 2 millimeters thick. Longer periods arerequired for thicker layers and vice versa.

At speeds of entry into the non solvent through an unstabilized surfacewith the polymer sheet uppermost or less than 10 to 15 feet per minutesurface patterns due to ripples are very likely to occur on the surfaceof the coagulated polymer layer. Until the material has been immersed innon solvent for at least 1/2 hour, it cannot be conveyed around a rollerwithout the material being damaged. The present invention enables lowspeeds for example 1 to 3, 5 or 10 feet per minute to be used resultingin shorter production lines and substantial savings in space and cheaperplant.

The applicants have discovered that many of these problems can beovercome if the layer of polymer extended with liquid vehicle isarranged uppermost on a support and liquid non solvent in continuousform applied thereto. This is carried out at least for long enough tocoagulate the immediate surface. The remainder of the coagulation can becarried out in the same or similar way even by spraying or by immersionin the conventional manner.

The applicants have also discovered that this application of non-solventcan be particularly conveniently carried out if a layer of polymersolution on a support is passed through a gap or slot and the gap orslot is sufficiently narrow for a stabilized meniscus to be set upbetween the upper surface of the layer and the adjacent wall of the slotwhen liquid non-solvent is supplied therebetween. The surface of thematerial can be coagulated in this way with a smooth surface and thecoagulation can be completed thereafter either by continued treatmentwith a thin layer of non-solvent held smoothly against the coagulatedsurface as by overlying spaced plates or trailing flexible sheets, or byimmersion of the layer in a conventional bath of non-solvent.

Moreover this enables the layer to be coagulated whilst supporteduppermost on a support, the support can thus easily be conveyed throughthe process and the conveying problems and the requirement for largetanks of non-solvent associated with upside down coagulation can beavoided.

The present invention also enables much smaller volumes of non-solventto be used for the coagulation and attendant removal of the bulk of theliquid vehicle or solvent. This reduces the scale of operation neededfor solvent recovery, allows the composition of the non-solvent liquidto be more readily controlled, and makes it possible should it bedesired for a wide range of non solvents such as alcohols glycols,ketones and other non solvent liquids other than pure water to be usedincluding those containing dissolved solids having non solvent actionsuch as inorganic salts for example ammonium compounds such as ammoniumnitrate. The smaller volumes required both reduce the capital cost ofthe non solvent and fire risks and other hazards.

Whilst the process and apparatus is particularly suited to theproduction of unreinforced microporous polymer layers its advantageswill also be obtained in the production of reinforced layers. Thus whena "sheet of polymer extended with liquid vehicle" or "polymer extendedwith liquid vehicle in sheet form" is referred to both unreinforced andreinforced polymer compositions are included within the term unlessotherwise stated. Thus the invention is applicable to the coagulation ofpolymer extended with liquid vehicle not only when the polymercomposition exists as a single homogeneous layer but also when it existsas a number of superposed layers of varying conpositions for examplevarying in polymer, pigment, stabilizer or other component nature orcontent, as also when the polymer composition forms a coating on or isimpregnated into reinforcing sheets for example of woven, non woven orknitted fibrous material and especially needle punched polymerimpregnated felts such as described in U.S. Pat. Nos. 3,100,721 and3,067,482.

Thus according to one aspect of the present invention a method of makinga water vapour permeable, preferably microporous, polymer sheet materialcomprises at least initiating the coagulation of polymer extended withliquid vehicle in sheet form, preferably arranged uppermost on asupport, by applying liquid non-solvent in continuous form to at leastone surface of the said sheet, desirably by forming a continuous,desirably thin, layer of liquid non-solvent extending across the widthof the sheet on the said surface of the said sheet, preferably by movingthe said sheet past means for forming the said layer. Preferably theapplication of non-solvent is carried on at least long enough tocoagulate the immediate surface of the layer i.e. so that a surface skinis formed and the layer is no longer sticky to the touch.

The liquid non-solvent is applied by means which are held out of contactor do not come into contact with the free surface of the polymer layer.Thus the surface remains flat and smooth and does not conform to theprofile of the means which apply the liquid non-solvent, i.e. thesemeans do not exert pressure on the surface of the layer such as toconform or modify the configuration of the surface.

According to another aspect of the invention a method of making a watervapour permeable preferably microporous polymer sheet material comprisescoagulating at least one surface of a sheet comprising polymer extendedwith liquid vehicle by causing continuous relative movement of the saidsheet with respect to the body of coagulating liquid, a surface of thesaid sheet coming into contact with the said body of coagulating liquidalong a line of contact bounding an exposed surface of the said body ofliquid while the said liquid surface is stabilized along a line spacedfrom the said surface of the sheet not more than 1 centimeter andpreferably in the range 0.1 to 8 millimeters especially 2 to 7millimeters.

Preferably the said liquid surface is stabilized by the presence of asolid boundary located along the said spaced line. Desirably thedistance between this said boundary and the surface of the sheet is suchas to enable a meniscus of non-solvent liquid to form therebetween.

The distance between the solid boundary and the surface of the saidsheet is desirably maintained substantially constant during the relativemovement which conveniently consists of the sheet being moved past thebody of liquid.

Reference has been to the relative movement being "continuous." The useof this term is not intended to mean that the movement persistsindefinitely but merely is carried out for substantial periods of timee.g. numbers of hours or days rather than minutes and more importantlythat it is not intermittent and is of substantially constant speedduring a given period of operation of the process. Clearly, however, thespeed can vary from run to run depending on the relevant factorsinvolved in the particular run.

The continuous layer of liquid non-solvent can be formed by a variety ofmeans through a preferred form of apparatus utilizing a close spacedplate and the surface tension of the non-solvent is described below.

In the preferred form of the invention the body of liquid can beconsidered as a layer located between the plate and the sheet. Howeverthe exact length of this layer of non-solvent in the direction ofrelative movement of the sheet and the means forming the layer ofnon-solvent, the machine direction, can vary though in the preferredembodiment this first layer is some 15 to 30 cms (6 to 12 inches) long.

This body of liquid may constitute a layer but should have a definedlinear upstream boundary preferably stabilized at least againstmacroscopic movement so that any movements in the non-solventsubsequently in contact with the sheet are substantially prevented fromreaching the said upstream boundary where the coagulation commences.

Whilst the plate and slot arrangement described below is found toproduce very satisfactory results and is preferred the body of liquid orlayer may not need to extend any finite length in the machine directionso long as a defined thin linear region of non solvent is established.Thus a bar extending across the sheet in close spaced relationship couldbe used as could be a porous pipe similarly spaced and fed with liquidnon solvent or a spaced calendar roll rotating with the same surfacespeed as the sheet in the same direction.

Also the non solvent could be fed evenly onto the surface from a slitextending across the sheet which would be inclined so that the liquidwould flow in the same direction as the sheet was moving. Thenon-solvent could also be thickened with suitable thickening agents andextruded onto the sheet, the thickening agents being removedsubsequently in the coagulation.

However the specific embodiments described hereafter are preferred tothese alternatives.

It has been mentioned above that the continuous layer of non solventused to initiate coagulation is desirably thin and that it is preferredthat the coagulation is completed by maintaining a substantiallycontinuous and again desirably thin layer of non solvent on at least thefree surface.

The thickness of the layer of non solvent is not thought to be criticaland can be as thin as desired in order, for example, to reduce thevolume of liquid from which the solvent will have to be recoveredprovided that it is adequate to meet the sheet's requirements to achieveeven coagulation as rapidly as possible.

Thus the layer does not need to be thicker than 1 centimeter thick andcan be as thin as 0.1 millimeter though at this thickness it may becomedifficult to maintain the layer in substantially continuous form. Auseful and readily achieved thickness for the non-solvent layer is inthe range 1 to 5 millimeters particularly when water containing up to10% w/w of dimethylformamide is used as the non solvent. A wetting agentmay usefully be added to the non-solvent to assist in the maintenance ofthe layer of non-solvent.

Another advantage of the use of a thin layer of non solvent rather thana bath of non solvent is that the concentration of liquid vehicle orsolvent in the non solvent more rapidly reaches a level at which solventrecovery is economically feasible and thus the non solvent need be usedfor shorter periods of time and can be kept more pure, than is the casewhen baths of non solvent are used, with attendant plant and processadvantages.

The support may be made of any material which is sufficiently flexibleto run through the process and has sufficient solvent and temperatureresistance to withstand the process conditions and also is such that thepolymer layer adheres to its surface at least sufficiently to produce aflat layer on the surface of the support.

The support can thus comprise woven, knitted or non-woven textilematerials, flexible polymer sheet materials and flexible metal materialssuch as wire gauze. The textile materials are preferably made frompolyester fibres though any other solvent resistant fibre could be used.

When woven supports having a regular array of holes are used the supportpreferably has the following characteristics. The support preferably hasa stiffness (as herein defined) in the transverse or weft direction inexcess of (S1/4) 0.6 and (S1/2) 0.9 preferably 0.8 and 1.3 andespecially 1.8 and 2.2 whilst being flexible in the longitudinaldirection, a support area (as herein defined) in excess of 50%,preferably 60% and especially 70 to 95% and at least 500 passages persquare inch, preferably 1000 and especially 5000 to 10,000, providingcommunication from face to face of the support.

Stiffness is measured using the three point beam loading method. Thespan of the sample between the two knife supports is 3.8 centimeters andthe load rate used is 10 millimeters per minute and is applieddownwardly to the centre of the sample. Stiffness (S1/4) is defined asthe load in Kgs required to produce a 1/4 centimeter deflection andstiffness (S1/2) is defined as the load in Kgs. required to produce a1/2 centimeter deflection.

Support area is defined as the % of the total area of the surface of thesupport which is occupied by the material of the support and is within0.5 millimeters of the surface. With woven fabrics there may be primaryand secondary support areas and the sum of these is the support area asdefined herein. Primary support is the area provided by the warp threads(and possibly also weft in a plain weave) at the highest points in thesurface and is the % of the total area provided by such threads above aplane passing through their mid points at the highest points in thesurface. Secondary support is the % of the total area less the primarysupport provided by all threads above a plane passing through the midpoints of the weft threads at their point of nearest approach to thesurface.

Thus in general terms primary support except in plain weaves is providedby the warp threads and secondary support by the weft threads. In plainweaves weft and warp both contribute and there is no secondary support.

The support can be porous or non-porous but if it is non-porous thesurface is preferably treated as by roughening it to produce a surfacekey for the polymer layer. Whilst the process is applicable to theproduction of permanent coatings on supports, it finds particularapplication in the production of unsupported sheet materials and in thiscase the support must be one from which the coagulated polymer layer canreadily be stripped without being damaged.

This stripping stage may result in a desirable drawing out of the innersurface of the material imparting a smooth fibrous appearance to ithaving in certain cases marked resemblance to the flesh surface of aleather produced from a natural hide, thus obviating the need for a shoelining when the articles are used as shoe uppers.

One particular material, which is both self supporting and has a degreeof flexibility and gives a very good flesh surface appearance, is aporous liquid permeable sintered polymeric plastics material especiallyone made from high density polyethylene and preferably having an averagepore size of 50 microns or more broadly 25 to 100 microns as measured bythe method described in B.S.S. 1752;1963 using n propyl alcohol. Such amaterial is sold under the name VYON (Trade Mark).

Preferably the support at least at its edges is such as to provide a keyfor the polymer sheet such that the force required to remove the sheetfrom the keyed area is below the breaking strength of theself-supporting sheet at the time at which the sheet is removed from thesupport, but in the range 50 to 2000/grams per centimeter width,especially 90 to 1400 grams per centimeter width.

The sheet can be removed immediately after coagulation or after beingleached when a filler is used or after being dried.

The degree of adhesion can be controlled by wetting the support beforethe layer is formed on it either with solvent or liquid vehicle or withnon-solvent liquids either pure non-solvent or non-solvent/solventblends having non-solvent action. Thus the support desirably is treatedso as to have an even non-solvent content prior to the applicationthereto of the sheet of polymer extended with liquid vehicle so as toensure even penetration of the support and that any coagulation causedby the non-solvent content of the belt is also even. The support maythus carry an amount of non solvent liquid prior to the application ofthe layer of polymer to it so as to ensure that in conjunction with thesurface properties of the support the layer of polymer adheres to thesupport without curling away from it but is still capable of beingstripped from the support without rupturing once the polymer has beencoagulated to self-supporting form.

The support preferably carried 50 to 80% by weight based on its ownweight of the non-solvent liquid. The non-solvent liquid may comprise 5to 40% of the liquid vehicle the balance being the liquid non-solvent.

Reference has been made to the polymer layer on the support being passedthrough a surface of liquid non-solvent which surface is stabilizedagainst movement. The word through is not to be taken as excluding thecase where the surface of the polymer layer forms one boundary of thestabilized surface and indeed this is the most convenient way ofcarrying the invention into practice.

Thus, according to a further aspect of the invention a method of makinga water vapour permeable polymer sheet material comprises forming aninitial mixture comprising polymer extended with liquid vehicle, forminga layer of the initial mixture on a support passing the layer on thesupport past coagulating means for establishing a thin continuous layerof non solvent across the polymer layer with a stabilized upstreamboundary and establishing such a layer of non-solvent on the surface ofthe said polymer layer. As mentioned above the invention also extends toapparatus for carrying out the process of the invention.

Thus according to this aspect of the invention apparatus for coagulatingpolymer extended with a liquid vehicle in sheet form on a support whichmay be porous or non-porous comprises first means for evenly applyingliquid non-solvent in continuous form across the full width of the saidsheet at least to the free surface of the said sheet irrespective of theorientation of the said sheet on the support, and drive means arrangedto move the said sheet on the support past the said coagulating means.Preferable the support is arranged to have the sheet on its uppersurface and the coagulating means are arranged to establish a continuouslayer of liquid non-solvent across the full width of the sheet.Desirably the coagulating means are such that the upstream boundary ofthe layer of liquid non-solvent is substantially stabilized againstmovement. Thus the coagulating or first means may utilize the surfacetension of the liquid non-solvent to stabilize its upstream boundary.

Thus the apparatus desirably includes coagulating means for establishinga thin continuous layer of liquid non-solvent across a surface whilstthe said surface passes the coagulating means, the said thin layer ofliquid having a stabilized upstream boundary in the sense of movement ofthe said surface, means for forming a layer of an initial mixture on asupport the surface of the layer remote from the said support comprisingthe said surface, and means for passing the layer on the support pastthe said coagulating means so that the said layer of liquid non-solventis established and the said surface of the initial mixture iscoagulated.

The first or coagulating means may comprise a plate positioned inclose-spaced relationship to the free surface of the sheet so as to forma slot therebetween and means for supplying non-solvent so as to keepthe slot filled. The thickness of the slot is desirably such that whennon-solvent is supplied to the downstream end of the plate the surfacetension of the liquid establishes a meniscus, adjacent to the upstreamend of the plate, between the plate and the free surface of the sheet onthe support.

The coagulating means preferably comprise a strip or plate arrangedtransverse to the direction of travel of the support and adjustablyspaced from the surface of the support so that a slot can be producedsuch as to enable a meniscus to be set up in the gap between the stripor plate and the surface of the polymer layer remote from the support. Afurther plate is preferably arranged beneath the support. Thenon-solvent supply means preferably comprise porous tubes extendingacross the width of the plate. These are preferably placed above thedownstream end of the plate.

They are conveniently made of the sintered high density polyethyleneVYON (Trade Mark) material described above.

They have the advantage of filtering the non-solvent supply andproviding an even feed and are cheap and can thus be easily replaced.

As mentioned above in a preferred form of this aspect of the inventionthe thin layer of non-solvent is maintained on the surface of the layerof polymer on the support until the said layer of polymer issubstantially completely coagulated to self-supporting form.

This is preferably achieved by arranging the support to run over anelongated tray with spaced plates or sheets arranged over the supportand a non-solvent supply arranged to maintain the said thin layer ofnon-solvent in the gap between the said plates or sheets and the saidsurface of the polymer layer. The sheets may be freely floating on thelayer of non-solvent and may be merely secured at their upstream end.

The non-solvent may be provided at a number of stations spaced down thetray with drain slots across the tray prior to the next inlet station.The arrangement can then run on a countercurrent basis by feeding theoutflow from each stage to the inlet of the preceding upstream stage.This arrangement enables a controlled concentration gradient of solventand non-solvent to be set up, and controlled at any desired level.

This enables the procedures disclosed in U.S. Pat. No. 3,208,875 to bereadily carried out. However, whilst these may be desirable for certainsystems of polymer extended with liquid vehicle as disclosed in thatdocument they are not essential to the satisfactory coagulation of thepolymer-removable filler-solvent pastes preferred for use in the presentinvention.

The plate may be at a slight angle to the horizontal so that the supportpasses downwardly beneath the plate, the angle being such that themeniscus can be established merely by supplying non-solvent on to thepolymer surface adjacent to the downstream edge of the plate.

Preferably means are provided to maintain the free surface of the saidsheet at least wetted with non solvent until the polymer issubstantially completely coagulated.

These means may comprise at least one covering sheet positioned abovethe polymer layer downstream of the plate so as to establish andmaintain a thin film of non-solvent evenly over the surface of thepolymer layer.

The method and means by which the layer of non-solvent is maintained canbe varied. Thus fine sprays or mists could be used to keep the surfacewet but care would have to be taken to ensure that the wetting was evenacross the full width of the layer and for its full length until fullycoagulated.

In another form of the invention the apparatus comprises meanscontaining a body of liquid non solvent, means for introducing the sheeton the support into the body of liquid non solvent at a substantialangle to the horizontal, plate means arranged to extend above thesurface of the body of liquid non solvent so as to produce a slotbetween the free surface of the polymer sheet on the support and theplate, the slot being of such dimensions that in conjunction with theheight of the plate above the still surface of the body of liquid nonsolvent any ripples which are produced in the body of liquid as by thepassage of the support into it are dampened out at the meniscus producedadjacent to the top of the plate between the free surface of the polymersheet and the plate.

In a preferred form of the invention means are provided to maintain boththe free surface of the said sheet and the free surface of the saidsupport at least wetted with non solvent until the polymer issubstantially completely coagulated.

The polymer is preferably dissolved in solvent but the term polymerextended with liquid vehicle is intended to cover systems in which thepolymer is in emulsion, colloidal or gel conditions as well as those inwhich it can reasonably be described as being in solution. Suchcolloidal or gel conditions are conveniently achieved by addition ofnon-solvent to a polymer solution. Examples of this method are disclosedin U.S. Pat. No. 3,100,721 and 3,190,765. However, any method, such asaddition of an electrolyte, as disclosed in U.S. Pat. No. 3,491,053,which reduces the solubility of the polymer in the solvent can be usedto achieve a colloidal or gel condition.

The disclosures of these four specifications are incorporated herein byreference.

The non-solvent is preferably removed, as by drying, subsequent to theremoval of the solvent, it merely being necessary that, whilst anysolvent remains such as could cause disadvantageous reduction inpermeability sufficient non-solvent remains to prevent this happeningfor example by using a non-solvent with a higher boiling point than thesolvent.

If the viscosity of the system is insufficient to enable sufficientlythick coatings to be formed it can be increased by cooling the mixtureor adding thickening agents or by other conventional means.

A high vapour permeability was mentioned above as being desirable inshoe uppers for certain uses. Whilst a degree of porosity can occur whena layer of a polymer solution is bathed with a non-solvent for thepolymer miscible with the solvent, the pores formed whilst impartingsome vapour permeability are liable to be not predictably or evenlydistributed and may vary widely in size depending on a wide range ofparameters.

An even fine pore size can be ensured by distributing evenly through thepolymer solution finely divided particles of a removable filler, forexample a solid powder of an inorganic salt, which remain solid in thepolymer solution or are arranged to be in a finely divided solid statewhilst the polymer is coagulated and are removed at that time orthereafter for example by a leaching agent.

Preferably the ratio in parts by weight of removable filler to workingmaterial in the initial mixture is in the range 1.5 to 1 up to 3.0 to 1preferably about 1.7 to 1 up to 2 to 1 and the ratio in parts by weightof working material to solvent in the initial mixture is in the range20:80 to 40:60, for example 25:75 to 35:65. Preferably the filler isground so that more than 50% of the particles have diameters in therange 4 to 20 microns.

It will be appreciated that the variation of the parameters of polymerconcentration and removable filler concentration for a given polymerconcentration, i.e. filler to polymer ratio will affect the propertiesof the article, an increase in filler to polymer ratio and a decrease inpolymer concentration tending to produce a more open i.e. more permeablestructure.

Thus, if it is wished to produce a strong essentially microporousmaterial with essentially no macropores visible to the unaided eye, forexample using a thermoplastic essentially linear polyurethane derivedfrom a polyester by reaction with a diol and a slight excess ofdiisocyanate dissolved in dimethylformamide and using sodium chloride asthe removable filler and water as the leaching agent, then the polymerconcentration is desirably in the range 25 to 35% w/w, especially about30% w/w and the filler to working material ratio is preferably in therange 1.5 to 1 to 2.0 to 1 in parts by weight.

If it is wished to produce a more open and plump material having asubstantial number of macropores which result in softer more resilientmaterial then using the same system the filler polymer ratio is selectedto be about 0.5 to 1.

It will be appreciated that the leaching agent for the filler does notneed to be the same as that for the solvent, thus for example when thesolvent is dimethylformamide, it could be removed by methanol and thefiller for example sodium chloride could be removed with water.

The polymer can be any organic resin material which is capable offorming a film on coagulation from an emulsion, a colloidal dispersion,a gel or a solution, whether the film is water vapour permeable or not.

The polymer must also be capable of undergoing the various processesspecified in the methods described below. However, when the product isintended for use as a man-made leather-like material an elastomericpolymer is preferably used. The particular strength and wearcharacteristics required for the end use of the man-made leather-likematerial will determine the particular polymer to be used.

For shoe uppers high abrasion resistance and tear strength combined witha reasonable extensibility and initial modulus to provide proper wearcomfort on the foot are required.

Many thermoplastic polymers can be used, for example polyvinylchlorideand its copolymers, acrylonitrile polymers and copolymers andpolyurethanes or blends of one or more of these.

The elastomeric polyurethane may be used on its own or as blends withminor proportions say up to 49% preferably less than 20% of polyvinylchloride and other polymers and copolymers such as nitrile rubbersincluding solid copolymers of butadiene and acrylonitrile.

Other polymers which have been suggested for use in man-madeleather-like materials include polyacetal resins, vinyl halide polymers(including copolymers with other ethylenically unsaturated monomers),polyamides, polyesteramides, polyesters, polyvinyl butyral,polyalphamethylstyrene, polyvinylidene chloride, polymers of alkylesters of acrylic and methacrylic acids, chlorosulphonated polyethylene,copolymers of butadiene and acrylonitrile, cellulose esters and ethers,polystyrene and other polymers made from monomers containing vinylgroups.

The preferred polymers however are elastomeric polyurethanes havingrecovery properties intermediate between pure rubbers and purethermoplastic materials at room temperature.

The article by Schollenberger Scott and Moore in "Rubber Chemistry andTechnology" Vol. XXXV, No. 3, 1962, pages 742 to 752 at page 743 and inFIG. 3 indicates the long so-called half lives of the polyesterurethanes made from adipic acid, 1,4 butane diol and diphenyl methane -p,p' -diisocyanate by the methods disclosed in U.S. Pat. No. 2,871,218and sold under the Trade Mark ESTANE 5740. These two disclosures areincorporated herein by reference.

Polyurethanes may be based on a wide variety of precursors which may bereacted with a wide variety of polyols and polyamines andpolyisocyanates. As is well known the particular properties of theresulting polyurethanes to a large extent can be tailored by suitablechoice of the reactants, reaction sequence and reaction conditions.

The preferred polymers are elastomeric polyurethanes based on a linear,hydroxyl terminated polyester (although a polyether or apolyether/polyester blend may be used) and a diisocyanate, with a smalladdition of a difunctional low molecular weight reactant. The lastmentioned component may be added either with the other reactants at thestart of a one-step polymerisation or at a later stage when it will actprimarily as a chain extender.

This type of polyurethane having thermoplastic properties isparticularly preferred for use in producing shoe uppers. Particularlypreferred polyurethanes are those derived from polyesters by reactionwith diols and diisocyanates. As is known from U.S. Pat. No. 2,871,218,mentioned above, many different polyesters, diols and diisocyanates canbe used, but a particularly suitable polyurethane system is one in whicha polyester made from ethylene glycol and adipic acid is reacted with1,4-butylene glycol and with 4,4'-diphenylmethane diisocyanate.

In the system in accordance with the above specification the mole ratioof polyester and diol can vary between quite wide limits but thecombined mole ratio of polyester and diol is arranged to be essentiallyequivalent to the mole ratio of diisocyanate so that the resultantpolymer is essentially free of unreacted hydroxyl or isocyanate groups.

Polymers of this type but having an improved Shore hardness can be madeby using a slight excess of diisocyanate and also be using a copolyesteras by replacing part of the ethylene glycol in the above system by1,4-butylene glycol.

A further alternative polyurethane system which has been foundparticularly suitable uses polyesters derived from caprolactones.

The polymers may be produced by a bulk polymerization process andsubsequently dissolved in suitable solvents or may be prepared directlyin solution by a solution polymerization process.

The polymer can include conventional stabilizers, fillers, processingaids, pigments, dyes, additives and surface active agents for exampleproofing or wetting agents.

Any material which is essentially insoluble in the solvent for thepolymer and is inert to the solvent and the polymer but which is solubleor can be rendered soluble by a liquid which has no deleterious effectson the polymer can be used as the removable filler. However, inorganicsalts such as ammonium sulphate and particularly sodium chloride arepreferred because of their ready availability and the ease with whichthey can be converted to a finely divided form. The use of suchtemporary fillers has the added advantage that the mixture containingsuch fillers has a substantially higher viscosity than the polymersolution or polymer extended with liquid vehicle and this facilitatesthe formation of thick layers. Whilst the preferred method ofcoagulating the polymer is to wash it or immerse it in a liquidnon-solvent for example water or water solvent blends for example of upto 40% or more dimethylformamide concentration any other liquidcoagulating method can be used which deposits a continuous though waterpermeable layer. Such other methods include using a liquid non-solventcontaining high concentrations of dissolved electrolytes, for example15% or higher aqueous sodium chloride solution. The removable fillermaterial is preferably microscopic particulate material which preferablycan be removed by dissolution or thermal decomposition. In place of ortogether with, the salt particles, other pore-forming microscopicparticulate material may be used. These particulate materials may bestarch particles (which may be removed by treating the coagulated layerwith an aqueous starch-digesting agent, such as an enzyme, of knowntype). Or they may be other microscopic solid particles which areinsoluble in the polyurethane solution at least at the stage when thepolymer is coagulated, for example urea, and which can either bedissolved out by treating the coagulated sheet with water or othersuitable solvent for the particles which is a non-solvent for thepolyurethane or can be otherwise destroyed or removed; examples of suchparticles are sodium carbonate, oxalic acid, ammonium carbonate, orsuitable microballoons. Alternatively the void-forming particulatematerial may be in the form of dispersed microscopic droplets of aliquid insoluble in the solution of polyurethane or in the form ofdispersed microscopic bubbles of gas.

Additionally the pore formation could be controlled by dispersingremovable or permanent fibrous fillers such as polyvinyl alcohol fibresor polyamide fibres or by incorporating permanent particulate fillers,such as silica, preferably a very small particle size for example lessthan 1 micron.

When a removable filler, such as sodium chloride, is used the pore sizeof the resultant water vapour permeable material depends to some extenton the particle size of the removable filler in the paste and if amicroporous material is to be made then none of the filler particlesmust be larger than 100 microns, and preferably most are substantiallysmaller, for example less than 50 microns typically in the range 1 to 40or 3 to 20 microns.

Many solvents are known for organic polymers and any suitable one can bechosen for the particular polymer used and preferably an organic solventis used. However, for elastomeric polyurethanes, N,N'-dimethylformamide,dimethyl sulphoxide, N-methyl pyrrolidone, dimethylacetamide andtetrahydrofuran are particularly useful. Dimethylformamide, dimethylsulphoxide and the other solvents can be diluted with other cheapersolvents such as toluene and methylethylketone which although notsolvents for polyurethanes on their own do not act as non-solvents whenmixed with dimethylformamide or the other solvents mentioned above.

The invention may be put into practice in various ways and two specificembodiments will be described by way of example with reference to theaccompanying diagrammatic drawings in which

FIG. 1 shows a longitudinal cross section schematic diagram of apparatusin accordance with the invention,

FIG. 2 shows a longitudinal cross section of a simple continuous processembodiment of apparatus in accordance with the present invention.

THE APPARATUS SHOWN IN FIG. 1.

This consists of an endless conveyor belt 51 of woven polyester fabric 3feet wide passing around an idler roller 52 and a driven roller 53 in aclockwise direction.

The dry belt is about 1.5 millimeters thick, weighs 107 grams per squarefoot and is made from polyester multifilament fibre (TERYLENE - TradeMark) in a close woven twill weave. It has a 0.2% stretch at 10 poundspull per inch width 1.2% stretch at 50 pounds and 6.6% stretch at 100pounds. Its breaking point is at 327 pounds at which stage the stretch21.7%.

These results were measured using an Instron tensile testing machine at1000% extension per minute as described below. The belt is a thick densebut flexible material.

The upper part of the belt runs through a gently inclined tray 54 about40 feet long. At the upper end of the tray the belt passes through aslot 55 formed between parallel glass plates 56 and 57. The lower plate57 is about 21/2 feet long and the upper plate 56 is about 6 inches longand is adjustably mounted so that the thickness of the slot 55 can bevaried. A spray 60 or tap connected to a supply of water of other liquidnon solvent is positioned just downstream from the upper plate 56. Fourcross bars 61 carrying trailing polyethylene sheets 62 about 6 feet longare positioned evenly across the tray the first one starting immediatelybelow the spray 60. A tank 63 to receive the liquid flowing off the endof the tray 54 is positioned below the roller 53. The apparatus alsocomprises a reactor vessel for preparing polyurethane in solution andpipework, storage tanks pumps and mixing apparatus for distributingmicroscopic salt particles, stabilizers and other processing aids evenlythrough the polymer solution to produce one or more pastes and thendelivering the pastes to one or more extrusion dies located adjacent tothe upper end of the tray 54.

The apparatus shown in FIG. 1 is provided with two 30 inches wideso-called coathanger extrusion heads 66 and 67 so that a single ordouble layer of paste can be formed on the belt as desired. The rate offeed of the paste to the heads is adjusted to give the required wetcoating thickness for example in the range 0.1 millimeters to 3.0millimeters for each layer e.g. 0.1 or 0.5 to 1.0 for the top layer and0.7 or 1.2 to 1.8 or 2.5 for the substrate layer. The head 66 will bereferred to as the substrate head and the head 67 will be referred to asthe topcoat head.

The apparatus is used as follows. A polyurethane is formed in solutionin dimethylformamide from a polyester by reaction with a diol anddiisocyanate under an inert atmosphere as described in more detail inExample 1. The polyurethane solution containing approximately 30% byweight of resin solids is mixed with microscopic sodium chlorideparticles about 2 (e.g. or in the range 1.0 to 2.5) parts by weight perpart by weight of resin and with small amounts of stabilizers andpigment. This substrate paste is evenly mixed and deaired and fed to thedie 66 to produce a layer of about 0.070 inches (1.8 mm) wet thicknesson the belt 54. A top coat paste is formed in similar manner butcontains 3 (e.g. in the range 2.5 to 5 or 6) parts of sodium chlorideper part of resin and is fed to the die 67 to produce a layer of about0.030 inches (0.76 mm) wet thickness on top of the substrate layer. Thebelt contains 50 to 80% preferably about 70% non-solvent liquid, e.g.water, by weight just prior to the first head 66. The composition ofthis liquid is usually predominantly water but it may contain from say 5to 40% by weight of dimethylformamide and preferably 10 to 20% byweight. The layers on the belt are then passed at 1 foot per minutethrough the slot 55. Water at 5° to 60° C. in this case about 15° C. issupplied to the spray 60 at a rate sufficient to maintain the gapbetween the top surface of the paste layers and the underside of theplate 56 filled with liquid. This gap is arranged to be about 1 to 3millimeters, in this case 1/16 inch or 1.6 millimeters thick so that astable meniscus is set up at the upper end of the slot and a smoothcoagulated surface produced. In the steady state this liquid is observedby sampling at the centre of the slot to contain about 5% by weight ofN,N'-dimethylformamide. The water supply is sufficient to maintain theslot full of water and also maintain a film of water over the wholesurface of the paste layers for the full length of the tray.

In this arrangement the belt is stopped after 35 feet of paste have beenspread on the belt and the belt held on the tray with the water supplymaintained for at least 70 minutes to complete the coagulation of thepolymer to microporous self-supporting form. The wet self-supportingsheet is then stripped from the belt and immersed for 2 hours in waterat 60° C. to reduce the chloride ion content to 1000 milligrams persquare meter or less thus removing substantially all the salt andsubstantially all the dimethylformamide. The material is then dried at98° C.

THE METHOD FOR USE WITH THE APPARATUS OF FIG. 1

The polyurethane polymers used were made in solution indimethylformamide from a polyester by reaction with a diol and adiisocyanate under an inert atmosphere.

The reaction was carried out in pure N,N'-dimethylformamide (139 BritishGallons) using 130 parts of an anhydrous adipic acid/ethyleneglycol/butane 1:4 diol copolyester having a molecular weight of about2000 acid value less than 2 and a hydroxyl number of about 53, soldunder the Trade Mark DESMOPHEN 2001 (by Bayer) and 28.38 parts ofanhydrous butane 1:4 diol and 0.1022 parts of dibutyl tin dilaurate, 108parts of pure 4,4'-diphenylmethane diisocyanate was then added and themixture cooled so as to keep the temperature below 50° C. When thereaction was substantially complete additional butane 1:4 diol was addedto react with the remaining free isocyanate groups and the temperatureheld at about 50° C. until the viscosity was about 2,900 poise at 24° C.This additional diol was 2.2 parts. Finally 4.7 parts of a 1:1methanol/N'N-dimethylformamide blend was added to react with any tracesof free isocyanate still present. The viscosity at this stage of asample at 24° C. was 3,900 poise, the solids content was 30%, theintrinsic viscosity was 1.04 and the Huggin's slope constant k of theviscosity number plot was 0.37. This solution will be referred to assolution S1.

A sample of solution S1 was then diluted to 15% resin solids and cast ina flat glass reservoir. The solvent was evaporated off slowly overseveral hours and finally more rapidly under vacuum at 50° C. to give asubstantially solvent free transparent film 0.2 millimeters thick.

The ultimate tensile strength was 544 Kg per square centimeter measuredby the method described in British Standard Specification No. 3144/1958using an Instron tensile testing machine.

The notch tear strength was 128 Kg/centimeter.

Table 1 gives the values of certain physical properties of other polymersolutions used in the Examples

                                      TABLE I                                     __________________________________________________________________________                              Ultimate                                                                           Notch                                                                              Initial                                        Viscosity   Huggin's                                                                           Resin                                                                             tensile                                                                            tear modulus                                   Solution                                                                           poise at                                                                            Intrinsic                                                                           Slope                                                                              Solids                                                                            strength                                                                           strength                                                                           (25%)                                     Number                                                                             25° C.                                                                       Viscosity                                                                           constant                                                                           %   Kgs/cm.sup.2                                                                       Kg/cm.                                                                             Kgs/cm.sup.2                              __________________________________________________________________________    S1   3900  1.04  0.37 30  544  128  72                                        S2   1960  1.08       32  646       62                                        S3   2300  0.87       32  534  144  62                                        S4   2260  1.11       32                                                      S5   3040  0.97       32.3                                                                              675  158  61                                        S6   2500  0.98       31.3                                                    S7   3800  1.02       33.5                                                    S8   2720  1.08       31.8                                                                              727  153  82                                        S9   2240  1.13       32.2                                                     S10 2680  0.86       32.2                                                                              627  137  60                                         S11       1.06       29.4                                                                              628  147  62                                         S12 1620  1.0        32.5                                                                              507  161  76                                        __________________________________________________________________________

The notch tear strength was measured on an Instron tensile testingmachine using the method described in British Standard Specification No.2782:3/308A/1965. The specimen is cut with a single stroke of a pressusing a knife-edge punch with cutting edges of the form and dimensionsshown in FIG. 308,108 of the above British Standard.

The jaws of the testing machine are set 5 centimeters apart and the endsof the specimen gripped in the jaws. The jaws are then separated at 100centimeters per minute and the load at which a tear is initiated at thenotch is recorded.

EXAMPLE I

A stabilized topcoat polymer paste containing evenly dispersedmicroscopic salt particles was made by mixing 121.1 parts of polymersolution S9 with 35.4 parts of N,N'-dimethylformamide 126.4 parts offinely divided sodium chloride and 22.8 parts of a carbon black pigmentmaster batch.

The resultant paste contained 25% resin solids and had a viscosity of87,000 centipoise at 25° C. measured on a Brookfield Viscometer.

The pigment master batch was made by dissolving in 32.2 parts ofN,N'-dimethylformamide 2.4 parts of STABAXOL a carbodiimide stabilizeragainst hydrolysis of the polyurethane, 3.2 parts of CYASORB U.V. 24,namely 2,2'-dihydroxy-4-methoxy-benzophenone, a stabilizer against ultraviolet ray degradation of the polyurethane and 0.8 parts of IRGANOX1010, namely tetrakis [methylene 3-(3',5',ditertiary-butyl-4'-hydroxyphenyl propionate] methane, an antioxidentfor the polyurethane, and stirring into this 8.0 parts of Rajah Blackcarbon black pigment and, then mixing in 38.8 parts of the polymersolution SI described above.

The Rajah Black carbon black pigment is made by the channel process byColumbian International and is stated by them to have an averageparticle size of 0.02 microns, a surface area of 156 square meters pergram, an oil absorption to produce a fluid paste of 11.3 milliliters pergram and to produce a stiff paste of 1.23 milliliters per gram, a carboncontent of 95.2%, and a volatile matter content of 4.8%.

The sodium chloride was ground in a pin and disc mill with airclassification to separate out fines and return oversize particles forregrinding. The sodium chloride powder before dispersing in the polymersolution typically had an average particle size of the order of 10 to 20microns usually about 13 to 17 microns with a standard deviation of theorder of ± 10 microns. This measurement was made by sedimentationmeasurements using a Photoextinction Sedimentometer manufactured byEvans ElectroSelenium Ltd., Model No. 41 used in accordance with themanufacturer's instructions based on papers by H. E. Rose in Engineeringof Mar. 31 and Apr. 14, 1950, and Nature of 1952, Volume 169, page 287.

This apparatus consists of a chamber in which the solid whose particlesize is to be measured can be dispersed ultrasonically in a liquid andits rate of settling measured optically. The change in transmission oflight by the dispersion with time is related to the particle sizes ofthe particles and the measurements of this change enable the averageparticle size to be calculated. It will be appreciated that thesesedimentometer experiments give an indication of the general order ofparticle size of the majority of the particles.

Shadow photography of typical samples of the ground salt has indicatedthat the salt particles have random rough irregular shapes includingquite elongated shapes as well as more compact cube or block shapes.

The dispersions typically contain a few particles having a maximumdimension as large as 70 microns but substantially all of the particlesare less than 40 to 50 microns, and most are less than 25 to 30 micronsim maximum dimension and have dimensions in the range down to 1 micronor so though a few may be even smaller. The salt is also selected tohave a low moisture content so that it does not cake, for example lessthan 0.5% and especially in the range 0.2- 0.4%. It may also have ananticaking agent added namely MICROCAL at about 1% by weight. MICROCALis a very fine particle size coprecipitated lime and silicate anticakingagent sold by Joseph Crosfield & Sons Ltd. The mixing and millingconditions are preferably carried out at relative humidities less than70% at 25° C. and preferably at about 50%.

A substrate paste was made from a blend of solutions S1 and S2 and wasmixed with 1.78 parts per part of resin of microscopic sodium chlorideof particle size similar to that used in making the topcoat paste, and apigment master batch similar to that used for the topcoat paste so thatthe finished substrate paste contained 0.5% carbon black by weight basedon resin.

The paste had a viscosity of 1720 poise at 25° C. The substrate andtopcoat pastes were then fed under pressure to the heads 66 and 67respectively at pump pressures of 62 pounds per square inch and 105pounds per square inch respectively with a belt speed of 1 foot perminute. The water supply to the spray 60 was mains water at 15° C.

The material had the following physical properties thickness 1.65millimeters; weight 748 grams per square meter; ultimate tensilestrength L 11.0, × 11.8 Kgs. per square centimeter, elongation at breakL 300%, × 365%; initial modulus (25% extension) L 3.2 Kg. percentimeter, ×2.8 Kg. per centimeter; notch tear strength L 5.3, × 4.8Kg. per centimeter; pore size 10.5 microns (maximum), 5.5 microns(average), hydrostatic head 106 millimeters of mercury and averagedensity (calculated) 0.45.

These physical properties were measured as described in Belgian Pat. No.732,482.

EXAMPLES 2 to 6

These were carried out in the same manner as Example 1 apart from thepolymer solutions used and the fact that in Example 5 distilled waterinstead of mains water was used as the non-solvent. Table 2 sets out thepolymer solutions used, the viscositites of the pastes, the pumppressures to the extrusion heads, the belt speed and the physicalproperties of the materials produced, and includes the values forExample 1 for comparison. The units used in Example 1 are also used inthe table.

                                      TABLE 2                                     __________________________________________________________________________    Example 1    2     3      4    5    6                                         __________________________________________________________________________    Substrate                                                                     paste                                                                         Polymer                                                                       solution                                                                              S2/S1                                                                              S6/S5/S4                                                                            S7/S3  S7   S7   S7/S8                                     Paste                                                                         viscosity                                                                             1720 1880  1360   1620 1620 1720                                      Pump pressure                                                                 psi     62   70    112    70   70   71                                        Topcoat                                                                       paste                                                                         Polymer                                                                       slutions                                                                              S9   S8    S7/S11/S12                                                                           S2/S10                                                                             S2/S10                                                                             S2/S10                                    Paste                                                                         viscosity                                                                             870  1520  790    670  670  670                                       Pump pressure                                                                 psi     105  118   176    53   53   50                                        Belt speed                                                                            1    1     2      1    1    1                                         Product                                                                       thickness                                                                             1.65 1.55  1.66   1.96 1.93 1.81                                      Weight  745  667   657    798  756  753                                       UltimateL                                                                             11.0 9.3   9.2    11.0 10.  9.6                                       tensile                                                                       strengthX                                                                             11.8 8.8   8.1    11.7 11.3 10.6                                      ElongationL                                                                           300  278   313    318  309  291                                       at breakX                                                                             365  300   318    340  330  312                                       InitialL                                                                               3.2 2.7   2.3    3.1  3.0  3.0                                       ModulusX                                                                              2.3  2.6   2.2    3.2  3.1  3.3                                       25%                                                                           extension                                                                     Notch tearL                                                                           5.3  4.5   3.8    5.3  4.8  5.4                                       strengthX                                                                             4.8  4.4   4.1    5.5  5.4  5.4                                       Pore (Av.)                                                                            5.5  5.1   5.1                                                        size (max.)                                                                           10.5 11.1  10.3                                                       Hydrostatic                                                                   head    106  95    93                                                         Water vapour                                                                  permeability              4740 4740 4060                                      Average                                                                       density                                                                       (calculated)                                                                          0.45 0.43  0.40   0.41 0.39 0.42                                      __________________________________________________________________________

Water vapour permeability was measured by the method described inBritish Standard Specification No. 3177/1959, but at 37° C. and anominal humidity gradient of 100% relative humidity.

These materials are all flexible and have a handle and drape closelyresembling a high quality calf grain leather. They can easily be madewithout carbon black and post dyed or different pigments or dyes couldbe incorporated in the polymer pastes.

They can be given a variety of surface finishes to make them resemblenatural leather finishes for example they can be converted to suedes bybuffing or sanding and the surface which has in contact with the beltand carries an impression thereof can be rendered similar in appearanceto the flesh surface of natural leather by sanding. The surfaces canalso be embossed for example by using a patterned foil.

A variety of surface modifying finishes can be applied to the top coator top surface of a single layer material for example to impart a grainleather like finish.

One very suitable finishing operation is a treatment of the uppersurface with fine droplets of a solvent and heating, in a manner topartially collapse the microporous structure along the surface and forma thin fused polyurethane skin thereon; materials so finished often havea series of tiny spaced depressions, partially lined with fusedpolyurethane material (e.g. about 2 to 15 microns in thickness), at saidsurface. Another suitable finishing treatment involves applying to theupper surface of the microporous material a thin top coat, such as anaqueous emulsion of a suitable polymer (e.g. an alkyl acrylate polymeror copolymer such as copolymer of butylacrylate with some 15% ofacrylonitrile and about 1-2% of itaconic acid, which can be cross-linkedon heating by the inclusion of unreaformaldehyde condensation product inthe emulsion, as is well known in the art); the amount of such polymermay be insufficient to close the pores, or sufficient to provide a verythin layer, less than about one micron in thickness, whereby theappearance of the material is improved without unduly decreasing itsability to transmit water vapour. The top coat may be a continuous layerwhich imparts a glossy "patent" finish to the material; thus one mayapply a conventional organic solvent solution of polyvinyl butyral mixedwith solvent soluble melamineformaldehyde resin, and evaporate off thesolvent and cure (cross-link) the mixture of these two resins.

The material could also be dyed with conventional dyes for polyurethanesfor example before the treatment with solvent spray or the applicationof the top coat or both.

In yet a further alternative form of colouring treatment a dye iscontained in the non-solvent in which the polymer is coagulated. The dyemay be an "Irgacet" dye as mentioned above and the non-solvent may be anaqueous alcoholic solution of the dye.

These finished materials can be readily made into shoe uppers and thusinto shoes by conventional techniques.

THE APPARATUS SHOWN IN FIG. 2

This consists of an endless conveyor belt 10 of woven polyester fabricas described for FIG. 1 passing around three rollers 12, 13 and 14 andover an inclined tray 11. The tray 11 is arranged beneath the upper partof the conveyor and supports it for a substantial part of its length.The upper end 16 of the tray is on the same level as the top of theuppermost 14 of the three rollers thus providing a horizontal upperstretch 15 of conveyor belt. The lower end 17 of the tray terminatesjust before the lower roller 12 and an end roller 13 is located at thesame level as the roller 12 so that the lower part of the conveyor beltis held in a horizontal plane. The rollers 12, 13 and 14 are arranged todrive the conveyor belt down the tray 11 around the end roller 12horizontally to and around the roller 13 up at a steep inclined angleand around the roller 14 and thence back to the top 16 of the tray 11.The lower part of the conveyor belt is arranged to be within a tank 20so that when liquid is placed in the tank the horizontal lower portionof the conveyor belt 10 will be beneath the surface of liquid. The tank20 also contains rollers 21 and 22 each placed respectively just beforethe rollers 12 and 13 in the direction of travel of the conveyor belt10. A guide roller 23 and a dancing roller are located outside the tank20 together with a rewind reel 25 supported on rollers 26 and 27, roller26 being driven by a further roller 28.

A pair of calender rollers 30 and 31 are positioned between the rollers13 and 14 and are arranged to be capable of calendering the conveyorbelt to varying degrees.

A pair of extrusion heads 35 and 36 are located at the upper portion 15of the conveyor 10 so as to be adapted to lay down superimposed layersof polymer paste on the conveyor belt. At the top end of the tray 11 aplate 40 is supported adjustably parallel to the tray 11 so as toprovide a slot of adjustable thickness but of substantially greaterdimension in the direction of motion of the conveyor than its thickness.The plates extend across the full width of the conveyor. Immediatelydownstream of the plate in the direction of motion of the conveyor thereis arranged a Vyon tube 42 to feed liquid on to the surface of the plate40 at its downstream edge. Immediately downstream of this Vyan tubethere is a further glass plate about eight feet long provided with Vyonfeed tube 44 adjacent its upstream edge. Bars and trailing sheets 46 asin the arrangement shown in FIG. 1 are spaced down the rest of the tray.Below the end of each sheet 46 the tray has a transverse drain slot 47.Immediately prior to each bar 45 but downstream of the preceding drainslot there is located an inlet pipe 48. The drained liquid may either bediscarded from the slots 47 or fed back to the preceding inlet 48 actingin effect as a counter current.

Preferably pure water is fed in at the end of the tank 20 adjacent theroller 13 and the liquid fed in counter current from the end of the tankadjacent the roller 12 to the lower end of the tray 11. The water fedinto the tank is preferably C. 40° to 80° C and at the other end of thetank its temperature is about 30° C. and its N,N'-dimethylformamidecontent about 3% by weight. The outflow from the tank is fed to the tray11 in counter current and the final outflow 47 from the tray 11 isarranged to contain up to about 10% of dimethylformamide which isrecovered in a solvent recovery plant. The outflow could easily bearranged to have a higher solvent content but it has been observed thatas the solvent content increases in the initial coagulating liquid thecoagulated surface tends to be less smooth and can take on a mattappearance.

Measuring means (not shown) are located between the rollers 14 and theextrusion heads 35 between the two extrusion heads 35 and 36 and betweenthe extrusion head 36 and the plate 40 so as to enable control of thethickness of the extruded materials to be achieved. This can be done bymeasuring either the thickness or the weight of the material passing themeasuring means.

The equipment described above can be used to produce a water vapourpermeable self-supporting sheet material as follows:

The conveyor belt 10 is set running at the desired speed, e.g. 1 to 5 or10 preferably 3 to 5 feet per minute, the tank 20 is filled with liquid,for example water, sprays 42 are supplied with liquid for example water,and the pressure between the rollers 30 and 31 is adjusted to give thedesired moisture content in the conveyor belt 10. The take-up driveroller 28 is synchronized with the speed of the belt 10. The materialwhich it is wished to coagulate comprising polymer dissolved ordispersed as a colloid or gel in a water miscible organic solvent is fedto one or both of the extrusion heads 35 and 36 depending on whether asingle layer film or a double layer film is to be produced. The rate offeed polymer to the heads is adjusted to give the desired thickness orthicknesses, for example in the range 0.3 millimeters to 3.0millimeters. The film carried on the conveyor belt 10 and adhering to itpasses between the plate 40 and the tray 11. The separation of the plate40 from the top surface of the extruded film is adjusted to be such thata stable meniscus of liquid is established at the upper end of the plate40. The film on the conveyor thus first comes into contact with thecoagulating liquid in a stabilised state and the surface is evenlycoagulated. With water, even coagulation is produced when this meniscusof 1/4 and preferably 1/16th inch or below in thickness. As thecoagulating liquid is absorbed into the film further fresh liquid passesup beneath the plate 40 to replenish the meniscus. The film surface isthus rapidly coagulated and coagulation of the rest of the layer orlayers is continued as the conveyor passes beneath the sprays 42 andthen under the further stabilised film of coagulating liquid provided bythe polyethylene sheet 44. This sheet minimises the production ofsurface irregularities which are observed to occur when it is not usedand the sprayed liquid is merely allowed to run down the surface of theconveyor under gravity. The at least partially coagulated film thenpasses round the roller 12 and beneath the surface of the liquid in thetank 20. It will be appreciated that the conveyor belt is now uppermostand the film is underneath it. At the rate mentioned above for theconveyor and with the dunk tank 50 feet long the film takes 35 minutesto pass from the roller 12 to the proximity of the roller 21. Withinthis time sufficient solvent has been displaced from the film for thepolymer to be in a self-supporting condition. The leading edge of thefilm can thus be detached from the conveyor and led round the roller 21and back through the tank for a further period of 35 minutes duringwhich a substantial proportion, if not all, of the solvent is displaced,depending on the thickness of the layer or layers. The film is then ledaround the roller 22 out of the tank 20 over the roller 23 under thedancing roller 24 which controls the tension in the self-supportinglayer between the rollers 26 and 27 and then on the reel 25 in aclockwise direction under zero tension. The factors of film thickness,the nature of the solvent and coagulating liquid, the conveyor beltspeed and the temperature of the liquid in the tank 20 and the length ofthe tank 20 should preferably be selected to ensure that substantiallyall of the solvent is displaced from the polymer film but if outsidefactors make it desirable the process may be run so that some solventremains and this can easily be removed in a further treatment of theself-supporting sheet material with a solvent miscible nonsolvent forthe polymer which may be the same as or different to the coagulatingliquid.

In an alternative form of the invention the tray 11 is dispensed with asis the roller 12 and they are replaced by roller arrangements whichdirect the conveyor belt into the liquid in the tank 20 at asubstantially vertical angle. The plate 40 is supported parallel to thedirection of entry of the belt into the tank and its upstream edge isarranged to be at a height greater than the maximum ripple heightencountered in the tank when running the apparatus. This once moreresults in a stable meniscus being established which is automaticallyreplenished by surface tension from the body of the liquid in the tank.The length of the plate is adjusted to damp down the motion of theripples so that the meniscus is held sufficiently stable for a surfacefree from dunk lines to be produced.

The function of calender rollers 39 and 31 mentioned above will now bedescribed in more detail.

Their function is to control the amount of coagulating liquid retainedin the belt when the process is running continuously. It is necessary todo this in order to achieve a balance between the need for sufficientadhesion of the polymer supplied from the extrusion heads to prevent thelayers distorting when coagulated and the conflicting need to strip theself-supporting film from the conveyor belt at the roller 21. If toomuch coagulating liquid remains in the belt 10 the adhesion of theextruded film will be insufficient and the shrinkage produced by theinitial coagulation of the top surface of the film at the plate 40 maybe such as to cause the edges of the film to separate from the conveyorbelt or delaminate. If too little coagulating liquid remained in thebelt, the adherence of the coagulated layer to the belt at the time itreaches the roller 21 could be too great and this could causedifficulties. Thus whilst the layer will be self-supporting at thisstage it will not yet have achieved the strength which it will haveafter further treatment and the stripping stage at the roller 21 mighteven cause damage to the layer.

The particular degree of moisture needed in the belt as it approachesthe extrusion heads will vary depending upon the factors mentioned abovein connection with the amount of solvent likely to remain in the belt atthe end of its passage through the apparatus but a balance can be struckbetween too great and too slight adhesion by trial and error.

According to a further aspect of the invention a method of making awater vapour permeable polymer sheet material which comprisescoagulating at least one surface of a layer of polymer extended withliquid vehicle on a support is characterized in that the methodcomprises causing continuous relative movement between the layer on thesupport and at least two bodies of liquid non solvent the sheet beingtransferred from going in contact with one body to being in contact withanother by being lead around guide means in a manner such that its freesurface is held out of contact with the said guide means and isprevented from assuming a concave transverse configuration.

Desirably the contact with the bodies of liquid non solvent is carriedon for sufficient time to coagulate not only the surface of the layerbut also to coagulate the layer to a form in which it can be strippedfrom the support.

The guide means preferably comprise a large diameter cylindrical orconvex guide surface such as a roll. By large diameter is meant asurface having a radius which is many times e.g. 10 to 50 or more timeslonger than the thickness of the layer on the support.

The layer is preferably also supported as by guides stentors or grips insuch a way as to prevent it assuming a convex transverse configurationat least during its contact with the first said body of liquid andpreferably throughout the period from its initial contact with the firstbody of liquid up to the stage at which it can be stripped from thesupport without rupturing.

The first body of liquid can be established in accordance with theaspect of the invention which uses a close spaced plate described aboveand this may be most convenient when only low speeds of operation e.g.up to 10 feet per minute are desired to be used. The second orsubsequent bodies of liquid can be established in the same manner

Alternatively the second body of liquid can be provided by a tank, orbath such as is described with reference to FIG. 2.

Subsequent bodies of liquid if used could conveniently involve the trayand trailing sheet arrangements described with reference to FIG. 7 andwhilst the slot and plate entry arrangement might be a convenient way ofsealing the entry end of the tray to prevent non solvent spilling outother means such as idler rollers underneath the support or a smooth lipto the tray could be used.

Alternatively the first body of liquid can be provided by a tank or bathof liquid into which the layer on the support can be lead by beingpassed round guide means with the free surface of the paste away fromthe guide means so as to have the support uppermost in the tank.

As described above the support can then be held flat by stentors, gripsor other means.

The layer on the support would then pass out of the bath around furtherguide means and up and over a series of one or more trays as describedabove. The layer once coagulated could be stripped from the support.

The invention also extends to apparatus for coagulating polymer extendedwith liquid vehicle in sheet form on a support which may be porous ornon porous which comprises first means for applying a body of liquid nonsolvent at least to the free surface of the said sheet second means forapplying a separate body of liquid non solvent at least to the freesurface of the said sheet, drive means arranged to move the said sheeton the support past the said first and second means in sequence andguide means for transferring the sheet on the support from the firstmeans to the second means in a manner such that the free surface of thelayer is held out of contact with the said guide means and is preventedfrom assuming a concave transverse configuration.

What I claim as my invention and desire to secure by Letters Patentis:
 1. A method of making a water vapor permeable polymer sheet materialwhich comprises forming an initial mixture comprising elastomericthermoplastic polyurethane dissolved in dimethylformamide withmicroscopic water soluble inorganic salt particles dispersed through themixture, forming a layer at least 0.5 mm thick of the initial mixture inadherence with an endless porous fabric belt, passing the layer inadherence with said belt into contact with a body of coagulating liquidwhich is a solvent for said salt while the free surface of said layer isuppermost and has coagulating liquid on and over it and while said layerand belt are so supported as to prevent said layer adhered to said beltfrom assuming a concave transverse configuration, said layer being incontact with said body for a time sufficient to partially coagulate saidlayer, said layer adhering to and becoming keyed to said belt, bringingsaid partially coagulated layer on said belt into contact, with saidfree surface lowermost, with a second body of coagulating liquidnon-solvent which is a solvent for said salt, said belt and layerpassing through said second body until said layer is coagulated toself-supporting condition, and then stripping said self-supporting layerin sheet form from said belt, said second body of liquid being situatedunder said first body of liquid, said layer and belt being transferredfrom said first body to said second body by leading said belt carryingsaid adhering layer around guide means in a manner such that said freesurface is held out of contact with said guide means and is preventedfrom assuming a concave transverse configuration.
 2. Process as in claim1 in which said porous fabric is a woven fabric.
 3. Process as in claim2 in which the degree of said keying is such that the force required toeffect said stripping is in the range of 50 to 2000 grams per centimeterof width.
 4. A method as claimed in claim 3 in which the guide meanscomprise a large diameter cylindrical or convex guide surface such as aroll.